1. Field of the Invention
The present invention relates to a flat type image-forming apparatus using electron-emitting devices, and a manufacture method of the image-forming apparatus.
2. Related Background Art
Recently, light and thin displays, i.e., the so-called flat displays, have received widespread attention as an image-forming apparatus to be used in place of large and heavy cathode-ray tubes. Liquid crystal displays have been intensively researched and developed as typical flat displays, but still have problems that an image is dark and an angle of the view field is narrow. Emission type flat displays in which electron beams emitted from electron-emitting devices are irradiated to fluorescent substances to generate fluorescence, thereby forming an image, are also known as ones expected to be substituted for liquid crystal displays. The emission type flat displays using the electron-emitting devices provide a brighter image and a wider angle of the view field than the liquid crystal displays. Demand for the emission type flat displays is increasing because they are also adaptable for achievement of larger screen size and finer resolution.
There are known two main types of electron-emitting devices; i.e., a hot cathode device and a cold cathode device. Cold cathode devices include, for example, electron-emitting devices of field emission type (hereinafter abbreviated to FE), of metal/insulating layer/metal type (hereinafter abbreviated to MIM), and of surface conduction type. Examples of FE electron-emitting devices are described in, e.g., W. P. Dyke & W. W. Doran, "Field Emission", Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, "Physical properties of thin-film field emission cathodes with molybdenum cones", J. Appl. Phys., 47, 5248 (1976).
One example of MIM electron-emitting devices is described in, e.g., C. A. Mead, "Operation of Tunnel-Emission Devices", J. Appl. Phys., 32, 646 (1961).
One example of surface conduction electron-emitting devices is described in, e.g., M. I. Elinson, Radio Eng. and Electronic Phys., 7, 1290, (1965).
In a surface conduction electron-emitting device, when a thin film of a small area is formed on a base plate and a current is supplied to flow parallel to the film surface, electrons are emitted therefrom. As to such a surface conduction electron-emitting device, there have been reported, for example, one using a thin film of SnO.sub.2 by Elinson cited above, one using an Au thin film G. Dittmer: Thin Solid Films, 9, 317 (1972)!, one using a thin film of In.sub.2 O.sub.3 /SnO.sub.2 M. Hartwell and C. G. Fonstad: IEEE Trans. ED Conf., 519 (1975)!, and one using a carbon thin film Hisashi Araki et al.: Vacuum, Vol. 26, No. 1, 22 (1983)!.
As a typical configuration of those surface conduction electron-emitting devices, FIG. 22 schematically shows the device configuration proposed by M. Hartwell, et al. in the above-cited paper. In FIG. 22, denoted by reference numeral 1 is a base plate and 33 is a conductive thin film made of a metal oxide formed by sputtering into an H-shaped pattern. The conductive thin film 33 is subjected to an energizing process called forming by energization (described later) to form an electron-emitting region 34. Incidentally, the spacing L between device electrodes 31, 32 is set to 0.5-1 mm and the width W' of the conductive thin film 33 is set to 0.1 mm.
In those surface conduction electron-emitting devices, it has heretofore been customary that, before starting the emission of electrons, the conductive thin film 33 is subjected to an energizing process called forming by energization to form the electron-emitting region 34. The term "forming by energization" means a process of applying a DC voltage being constant or rising very slowly across the conductive thin film 33 to locally destroy, deform or denature it to thereby form the electron-emitting region 34 which has been transformed into an electrically high-resistant state. In the electron-emitting region 34, a crack is produced in part of the conductive thin film 33 and electrons are emitted from the vicinity of the crack. Thus, the surface conduction electron-emitting device after the forming by energization emits electrons from the electron-emitting region 34 when an appropriate voltage is applied to the conductive thin film 33 so that a current flows through the device.
The surface conduction electron-emitting device is simple in structure and easy to manufacture, and hence has an advantage that a number of devices can be formed into an array having a large area. Therefore, the application of the surface conduction electron-emitting device to charged beam sources, displays and so on have been studied in view of such advantageous features. As one example of applications in which a number of the surface conduction electron-emitting devices are formed into an array, there is proposed an electron source that, as described later in detail, the surface conduction electron-emitting devices are arrayed in parallel, i.e., in the so-called ladder pattern, and opposite ends of the individual devices are interconnected by two wirings (called also common wirings) to form one row, followed by forming this row in a large number (see, e.g., Japanese Patent Application Laid-Open No. 64-31332).
The applicant has previously proposed a flat type image forming apparatus wherein a base plate (hereinafter referred to also as a rear plate) including electron-emitting devices formed thereon and a base plate (hereinafter referred to also as a face plate) including a fluorescent film formed thereon are disposed to face each other, a space defined between both the base plates is evacuated into a depressurized state (or a vacuum state), and electron beams emitted from the electron-emitting devices are irradiated to the fluorescent film to form an image (see, Japanese Patent Application Laid-Open No. 2-299136).
FIG. 23 schematically shows a section of the above flat type image forming apparatus using the electron-emitting devices. In FIG. 23, the apparatus comprises a rear plate 1, electron-emitting devices 54, and a pressure bearing member 3 endurable against the atmospheric pressure. Denoted by 4 is a face plate on the undersurface of which a fluorescent film 5 and a metal back 6 are formed. An outer frame 8 is connected to the face plate 4 and the rear plate 1 through frit glass 7 in a sealed manner to construct an envelope (vacuum container). An inner space in the envelope is evacuated through a vent tube (not shown) to establish a depressurized state (or a vacuum state).
However, it has been found from studies made by the inventors that there is still room for improvement of the above image forming apparatus in points below. The presence of the pressure bearing member endurable against the atmospheric pressure in the vacuum envelope reduces evacuation conductance. Therefore, a relatively long time is required to evacuate the inner space of the envelope. Also, when the envelope is evacuated in a relatively short time, there arises a fear that the inner space of the envelope may not be sufficiently depressurized and a finally reached vacuum level may be relatively low. Accordingly, the operation of evacuating the envelope takes a larger percentage in the production cost. It is thus concluded that reducing the time required for evacuating the envelope greatly contributes to cut down the cost. Also, this effect is expected to become more remarkable in an image-forming apparatus having a larger display screen size.